[glibc] math: Add fast-path to fma

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=dfb150f3aadd93118a78ef9e19c2663f0e1994b7

commit dfb150f3aadd93118a78ef9e19c2663f0e1994b7
Author: Adhemerval Zanella <[email protected]>
Date:   Mon Mar 9 17:11:12 2026 -0300

    math: Add fast-path to fma
    
    For normal numbers there is no need to issue scalbn, the fma can set
    the exponend directly.   Performance-wise on x86_64-linux-gnu without
    multi-arch it shows a latency improvement of ~5% and throughput of %7
    (and sligth more for ABIs witht tail-call optimization).
    
    Checked on x86_64-linux-gnu and i686-linux-gnu with --disable-multi-arch.
    Reviewed-by: Wilco Dijkstra  <[email protected]>

Diff:
---
 sysdeps/ieee754/dbl-64/s_fma.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c
index d65d505915..88bda86943 100644
--- a/sysdeps/ieee754/dbl-64/s_fma.c
+++ b/sysdeps/ieee754/dbl-64/s_fma.c
@@ -192,7 +192,10 @@ __fma (double x, double y, double z)
     i = -i;
   double r = convertfromint64 (i); /* |r| is in [0x1p62,0x1p63] */
 
-  if (e < -1022 - 62)
+  if (__glibc_likely (e >= -1084 && e <= 960))
+    /* Fast-path for normal numbers.  */
+    return asdouble (asuint64 (r) + ((int64_t) e << MANTISSA_WIDTH));
+  else if (e < -1022 - 62)
     {
       /* Result is subnormal before rounding.  */
       if (e == -1022 - 63)